The present invention relates to a toothed belt for a power transmission device, and more particularly pertains to a new and improved toothed belt for driving a cam shaft of an automobile engine.
Conventionally, a toothed belt for driving a cam shaft of an automobile engine (hereinafter referred to as "toothed belt") comprises a rubber body made of a raw material of rubber, which is mainly composed of chloroprene. Recently, as automobiles have oriented high-class and high-efficiency, automobile engines are required to be high-powered. As a result, the conventional toothed belt, a rubber body of which is made mainly of chloroprene, lacks thermal and high-load resistance, because the ambient temperature rises in the place where the toothed belt is used, and also a cam shaft drive is highly loaded. On the other hand, other toothed belts have been developed in which the rubber bodies are made mainly of such materials as chlorosulfuric polyethylene or sulfur-crosslinked, hydrogenated nitrile. These belts, however, do not yet provide the desirable thermal and high-load resistance, because the high-power of an automobile engine has advanced beyond the advances in these toothed belts.
Further, it has hitherto been known as described in, for example, Japanese Patent Application Laid-open Nos. Hei 1-269743 or 5-164194, to provide a toothed belt including a rubber body which uses a carboxyl-terminated polybutadiene containing a peroxide crosslinking-based hydrogenated nitrile rubber composition as a co-crosslinking agent.
The actual situation with today's change of life-style, however, is that the owners of automobiles usually do not perform daily inspections thereof, and those owners cannot be expected to take preventive measures against the breakdown of their own automobiles. Under the circumstances, improvement has been requested in the efficiencies of the parts for the automobiles or in maintenance-free parts, not excepting the toothed belt. Although various attempts have been made with respect to the improvement of the toothed belt, the actual situation is that a life span of the toothed belt by itself has not been so prolonged as that of the automobile as a whole.
Various attempts have been made to provide a rubber composition adapted to a tooth rubber or back rubber of the toothed belt. Japanese Patent Application Laid-open No. Sho 63-270753 describes a technology utilizing a metal salt of organic peroxide and carboxylic acid as a crosslinking agent, thereby improving intensity characteristics of polymer. Japanese Patent Application Laid-open No. Hei 1-146935 describes a technology improving a modulus of a rubber by adding unsaturated carboxylate to ethylenic unsaturated nitrile conjugated diene-based highly saturated polymer rubber.
However, when repeated compressive deformation is applied to the toothed belt, which has been cured and formed by using the foregoing rubber composition, not only generation of heat but also compressive permanent strain grows, and the gears do not engage smoothly with a sprocket, so that a noise or a breakdown, such as tooth breakage, may occur. Further, when the latter rubber composition is used, the toothed belt becomes easily cut due to an insufficient bending fatigue limit thereof.